BRUSSELS — European Council President Donald Tusk on Tuesday lobbed a verbal grenade at President Trump ahead of a crucial week of diplomacy, warning the U.S. leader not to mix up his friends and his rivals as he meets with NATO leaders, then Russian President Vladimir Putin.

The searing criticism came as Trump plans to fly to Brussels later Tuesday, vowing to punish NATO allies for their failures to live up to their defense spending commitments. Trump has been far friendlier to Putin, saying the men have a range of issues to discuss and that he’s “fine.”

“I would like to address President Trump directly, who for a long time now has been criticizing Europe almost daily for, in his view, insufficient contributions to the common defense capabilities,” Tusk said after signing a cooperation agreement between the European Union and NATO.

“I want to dispel the American president’s argument, which says that the U.S. alone protects Europe against our enemies, and that the U.S. is almost alone in this struggle,” he said.

He told Trump to keep in mind European deaths in the conflict in Afghanistan, which NATO was pulled into after the United States became the only nation in history to invoke the alliance’s collective security provisions following the attacks of Sept. 11, 2001.

Tusk told Trump to remember that as he sits down with Putin on Monday.

“It is always worth knowing who is your strategic friend and who is your strategic problem,” Tusk said.

He suggested Trump had little regard for U.S. allies.

“Dear America, appreciate your allies. After all, you don’t have that many. And dear Europe, spend more on your defense, because everybody expects an ally that is well-prepared and equipped.”

European Council President Donald Tusk holds a joint news conference on July 10. (Aris Oikonomou/AFP/Getty Images)

Shortly after Tusk's fiery warning, Trump told reporters he thought the Putin meeting may indeed be warmer than the NATO summit.

“Frankly, Putin may be the easiest of all,” he said as he boarded Marine One on his way to Europe.

The remarks came after a Tuesday morning Twitter storm in which he repeated his frequent charge that Europe was taking advantage of the United States.

“Getting ready to leave for Europe. First meeting - NATO. The U.S. is spending many times more than any other country in order to protect them,” Trump wrote. “Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!”

“NATO countries must pay MORE, the United States must pay LESS. Very Unfair!” Trump wrote in a second tweet.

Tusk’s criticism further entrenched him as a European leader who has embraced a role as a foil to Trump. In May, he asked: “With friends like that, who needs enemies,” frustrated with Trump’s attitude toward Europe on trade issues.

“Dear President Trump: America does not have and will not have a better ally than Europe. Today Europeans spend on defense many times more than Russia and as much as China,” Tusk said Tuesday. “This is an investment in common American and European defense and security, which can’t be said with confidence about Russian or Chinese spending.”